Sulphonic acids of aromatic diisocyanates or polyisocyanates are known. They may be prepared in a simple manner by reacting the corresponding aromatic diisocyanates or polyisocyanates with sulphonating agents, such as sulphur trioxide, adducts of sulphur trioxide, oleum, chlorosulphonic acid or sulphuric acid (see, for example, German Offenlegungsschrift Nos. 2,227,111 and 2,359,615 and U.S. Pat. No. 3,826,769).
The products obtained may be solid, resinous or pulverulent sulphonation products or solutions of the sulphonated isocyanates in unreacted starting isocyanate, depending on the isocyanate used and the degree of sulphonation.
Whereas the handling of liquid sulphonated polyisocyanates entails no difficulties, considerable problems arise in the preparation, storage and use of solid isocyanato sulphonic acids in powder form.
In many cases, these products are obtained in such a finely divided form that they are difficult to separate and purify from the liquid reaction medium. The dried products raise dust when being put into containers or transferred. The most serious problem, however, is that dry powders prepared in this way are not stable in storage. The melting point and decomposition point rise during storage and the products become progressively more insoluble in organic solvents, as well as in the polyesters, polyethers and polyols normally used for the preparation of polyurethanes.
Although instability in storage is a general problem with polyisocyanates and is therefore well-known to those skilled in the art, in the case of solid pulverulent isocyanato sulphonic acids the deterioration in the quality sets in within such a short time after their preparation, for example, in some cases, after only a few days, that commercial production of usable polyaddition products is thereby rendered very difficult if not impossible.
On the other hand, there is a technological and economic demand for using polyisocyanato sulphonic acids in polyaddition chemistry in addition to or instead of the conventional diisocyanates and polyisocyanates because they are excellent starting materials for the production of hydrophilic polyurethanes, in particular polyurethanes which are water dispersible, and because they are very suitable physiologically and from the point of view of industrial hygiene since they have no vapor pressure and give rise to water-soluble amino sulphonic acids on decomposition. The preparation of polyurethanes from sulphonated tolylene diisocyanate has hitherto been carried out either by sulphonating the diisocyanate prepolymer instead of the pure diisocyanate (U.S. Pat. No. 3,826,769) or by preparing the isocyanato sulphonic acid only shortly before it is converted to the polyurethane (U.S. Pat. No. 3,826,769). The first method has the disadvantage of limiting the choice of sulphonating agents since, for example, sulphur trioxide causes decomposition phenomena when reacted with polyether prepolymers. If sulphuric acid is used as sulphonating agent, the resulting reaction is inevitably accompanied by chain-lengthening with formation of urea groups. Moreover, this method may only be used for sulphonating completely or partly free isocyanates, but not for sulphonating products in the form of urethanes. This means that, in the case of an isocyanate prepolymer, only the isocyanate end groups may be sulphonated. The second method cannot be carried out on a technical scale since the polyurethane manufacturer cannot be expected to carry out a process of isocyanate sulphonation prior to preparing the polyurethane.
It has also been proposed (U.S. Pat. No. 3,826,769) to dissolve the isocyanato sulphonic acids in an organic solvent, such as acetone, immediately after their preparation and to use them as solutions. This method is also impracticable since, for example, a solution of sulphonated tolylene diisocyanate in acetone is stable for at the most a few hours, after which it rapidly becomes cloudy and forms a precipitate.
It is therefore not surprising that solid isocyanato sulphonic acids have not hitherto become established in industrial use.
The problem therefore existed of preparing isocyanato sulphonic acids or stabilizing them in such a manner that they could be stored and used without problems and that solutions thereof in organic media would have a long storage life.